(a) A person who has membership in two or more retirement systems to which this chapter applies is subject to the laws governing each of those systems for determination of the person’s eligibility for service retirement benefits from each system, except that, for the purpose of determining whether a person meets the length-of-service requirements for service retirement of a system, the person’s combined service credit must be considered as if it were all credited in each system.
(b) A person’s combined service credit is useable only in determining eligibility for service retirement benefits and may not be used in determining:
(1) eligibility for disability retirement benefits, death benefits, or any type of benefit other than service retirement benefits; nor
(2) the amount of any type of benefit.

(c) A person receiving service retirement or lifetime disability retirement benefits from one or more retirement systems to which this chapter applies may use the program provided by this chapter to qualify for subsequent service retirement under another retirement system to which this chapter applies in which the person has service credit, if the person was not eligible to retire under the latter system at the time of previous service retirement, or qualification for lifetime disability retirement benefits from a retirement system to which this chapter applies, or if the person’s previous retirement was not based on combined service credit.
(d) Service credit earned with or allowed by more than one retirement system to which this chapter applies for the same service may be counted only once in determining the amount of a person’s combined service credit.